see comment above, but in short twitter need not hire cloudflare for services. Cloudflare plays an important role in the backbone of the internet. If twitter is resolved through cloudflare’s DNS, cloudflare can estimate the volume of traffic to Twitter.com. This appears what had happened given the published graph is “DNS ranking”. FYI, and for mine as well as I just discovered, we can generate that graph by our selves by using cloudflare’s Radar API